The proposed studies will seek to determine the mechanisms of gonococcal uptake of heme both as free heme and from hemoglobin (Hb). The HpuA/B Hb receptor is known to undergo phase variation from Hb+ to Hb+, and we will determine whether there is in vivo selection for the Hb+ phase in the female (menstrual) genital tract. The vaccine potential of HpuA/B will be assesses in many ways, including determination of variability in protein sequence, and the functional activities of antibodies against each of HpuA and HpuB. Surface exposure of HpuA and HpuB in whole cells will be determined, and we will investigate functional domains in each protein involved in Hb binding. Male human volunteers will be used in experimental urethral infection to determine whether expression of the HpuA/B receptor is sufficient to initiate infection at this site. Finally, a variety of genetic experiments are proposed to identify a putative separate receptor for free heme, and the genes involved in the heme transport pathway. These experiments are important to understanding the molecular pathogenesis of gonococcal infection, and will be of considerable relevance to analogous studies of other mucosal pathogens. The expected results will play a major role in determining whether these common gonococcal outer membrane proteins are vaccine candidates.